Preheat oven to 325°F. Spray a 9x5-inch loaf pan with nonstick cooking spray. Set aside.
In a medium bowl, whisk together the all-purpose flour, cake flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar on medium-high speed until light and fluffy, about 5 minutes.
Reduce the mixer speed to low and add the eggs one at a time, mixing until each egg is fully incorporated. Add the vanilla extract and mix until combined.
Gradually add the flour mixture, mixing on low speed just until combined.
Remove the bowl from the mixer and gently fold in the zucchini by hand, making sure it is evenly distributed throughout the batter.
Spread the batter into the prepared loaf pan.
Bake for 80 to 90 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs, but no wet batter. If the top begins browning too quickly, loosely cover the loaf with aluminum foil during baking.
Let the pound cake cool completely before adding the glaze.
To make the vanilla glaze, whisk together the confectioners' sugar, vanilla extract, and 3 tablespoons of milk until smooth. Add additional milk, a little at a time, until the glaze reaches your desired consistency.
Drizzle the glaze over the cooled pound cake before slicing and serving.